The present invention relates to a load control link and more particularly to a device which is capable of preventing overstressing under tension by limiting or controlling the maximum dynamic load through absorbing energy.
It is generally acknowledged that mooring loads experienced at single point moorings are related to the energy stored in the mooring system. This energy arises from the motions of the moored vessel in response to waves. A mooring system which can absorb a substantial amount of energy will experience lower loads than a relatively stiff mooring system. It is common to provide a mooring design with soft elasticity characteristics in order to maximize energy absorption and thus minimize the mooring loads experienced. The ability to provide elasticity is somewhat restricted by water depth and the properties of mooring hawsers. The ideal mooring system would have a load deflection curve with a constant slope up to the safe working load and a zero slope beyond that load point. A very complicated and elaborate servo-control system conceivably could provide such a load deflection curve; however, the use of such a system would most likely be impractical as part of a mooring system from cost, design and operability standpoints.
Prior art devices employed for controlling and limiting loads through the absorption of energy are disclosed in U.S. Pat. Nos. 3,096,268, Lindsey et al, 3,450,233, Massa 2,877,170 Greenhalgh et al, and 682,536, Duncanson.
Lindsey et al illustrate an energy-absorbing device which employs a shaft, a body, a sleeve attached to the body, and a cutter mounted on the shaft and bearing against the sleeve. The sleeve is cut as the cutter is forced along it. As the assembly is compressed due to impact, the kinetic energy of the body is dissipated and the body decelerates to rest. This device is unsatisfactory to serve as a load control link in tensioned lines because it absorbs impact energy and absorbs it in compression rather than in tension. Further, this device cuts metal from the inside of a sleeve which thus forces the sleeve to expand against the outer wall and makes removal difficult.
Greenhalgh et al is similar to Lindsey et al with the exception that the cut is made directly on the inside wall of the body instead of on a sleeve. The body must be of a material which has both strength and cutting properties. Further it must be replaced after each cut.
The Massa reference illustrates an energy absorbing device wherein a cutter comprising a disc is housed within the body and cuts along a shaft as the shaft moves axially due to a compression impact load imposed thereon. This cutting action will absorb energy. This particular device is deficient because it works in compression instead of tension. Further in this device the entire shaft must be replaced after each impact.
Finally, Duncanson illustrates a mooring system which includes a spring link for absorbing energy as the load increases, so that the chain does not break as the load increases. This system affords only a limited amount of load energy absorbing ability in that the spring begins to compress at low loads.
In spite of the prior art efforts and attempts at providing energy absorbing devices for controlling and restricting maximum dynamic loads, there still exists the need for a relatively simple device which will effectively limit maximum loads in and prevent overstressing of tension members by absorbing some of the energy in such loads.